Chapter 5 - Joins

Abstract

Structured query language (SQL) allows deriving a single table that contains all of the desired data. One can then query that table using the various SQL techniques already known. Combining tables to derive a new table is called a join. This chapter provides numerous examples of join queries (how to join two tables). The FROM clause creates the new table that has the combined attributes of all tables in the list. The table name prefix is not required if the attribute name is unambiguous, as with name in the SELECT clause. The WHERE clause describes how to connect the tables. After executing the FROM and WHERE clauses, a table is formed with the attributes of both the tables and the rows with matched data. Finally, SQL executes the SELECT clause, keeping only the attributes in the SELECT list. Once the new table is created, one can use it just like any other table.